1. Field of the Invention
The present invention relates to devices used to obtain a DC voltage, the value of which is adjustable over a wide range of values. Said devices are more particularly suited to providing the bias needed for focusing an X-ray beam, at a value that is chosen by the practitioner who uses the radiological installation.
A radiology tube is generally constituted like a diode, that is, it is constituted by two electrodes, one of which, called a cathode, emits electrons while the other, called an anode receives these electrons on a small surface that constitutes the source of X-radiation.
The cathode has a filament heated by an electrical current that constitutes the electron source. When a high voltage, given by a generator, is applied to the terminals of both electrodes so that the cathode is at a negative potential, a so-called anode current is set up through the generator and crosses the space between the cathode and the anode in the form of an electron beam.
To focus the electron beam, a metal element, called a focusing element, supporting the filament, is insulated from this filament and taken to a negative potential, called a bias potential, that is negative with respect to said filament. Furthermore, to modify the shape and hence the focusing of the electron beam, it is the common practice to modify this bias voltage over a wide range of values, for example between 300 and 3,000 volts. Besides, it must be noted that the cathode is itself taken to a voltage of the order of -20 to -75 kilovolts with respect to the ground. This raises problems of insulation in the application of this bias potential or voltage.
The invention more particularly relates to a device used to obtain a bias voltage for a focusing element of an X-ray tube cathode that can be made to vary over a wide range of values.
2. Description of the Prior Art
Such devices are known and, by way of indication, FIG. 1 shows a schematic diagram of a prior art device. This device has a supply circuit 10 that gives a regulated and adjustable DC voltage E from an AC voltage supplied by the mains. The voltage E is applied to the terminals of a DC/AC converter 11 that comprises a chopper circuit 12 and a control circuit 14.
The AC signal given by the DC/AC converter 11 is applied to a voltage step-up transformer 15, the secondary winding of which is connected to a rectifying and filtering circuit 16. This circuit 16 gives a DC voltage V.sub.s that is applied between the focusing element and the filament of the X-ray tube.
It must be noted that, since the voltage V.sub.s is difficult to measure owing to the high potential of the common mode (20 to 75 kilovolts), it is preferable to measure the voltage E which is substantially proportional to it and to regulate it. To this end, the voltage E is measured by a resistive divider comprising the resistors R1 and R2 and the divided signal is applied to a voltage/frequency converter circuit 20 which furthermore receives a signal V.sub.ref corresponding to the voltage that is to be obtained between the focusing element and the filament of the X-ray tube. The converter circuit 20 gives pulses of adjustable frequency and/or adjustable duration. These pulses activate the switches of the supply circuit 10 so as to modify the output voltage E and hence modify the voltage V.sub.s to obtain V.sub.s =V.sub.ref.
In a standard way, the chopper circuit 12 includes, for example, two transistors 21 and 22, the opening and closing of which are controlled by the control circuit 14.
The control circuit 14 is also a voltage/frequency control circuit similar to the circuit 20 but with a fixed frequency.
The drawbacks of this prior art device that has just been described are that:
- it necessitates two power converters: the first power converter circuit 20 to regulate the voltage E and the second power converter circuit 14 to obtain an AC voltage.
- it switches over the current in the semiconductors abruptly, and this is a source of parasitic phenomena;
- it has a low range of output voltages, for the voltage E that is adjusted cannot tend towards zero because of the limitations in the cyclical ratio of the chopper circuit.